The efficient use of composites in many fields of technology is due to a number of their valuable properties.
Such composites as, e.g. hard alloys exhibit a high hardness (86-94 HRA) in combination with high wear resistance, i.e. high resistance to wear in friction with both metals and nonmetals.
Another type of composites--carbide-containing steels-features a hardness which is close to that of cermets and a strength which is almost identical to that of steel.
Certain composites are capable to a large extent of retaining their properties at high temperatures.
It is most advantageous to use composites for the manufacture of cutting tools, dies and other products.
There are many diverse methods for making composites.
A most widely used method for making hard alloy materials (V. I. Tretiyakov "Fundamentals of Physical Metallurgy and Manufacturing Processes for Making Sintered Hard Alloys" (in Russian), 1976, Metallurgia Publishing House, Moscow, p.7), comprises preparing high-melting compounds with subsequent employment of powder metallurgy techniques comprising blending poweders of the resultant high-melting compounds with a binder metal, pressing compacts and sintering them at 1350.degree.-1550.degree. C. for several hours in vacuum or hydrogen electric furnaces.
High-melting compounds for hard alloys (carbides, borides, nitrides of transition metals) are produced by synthesis from respective metals (or their oxides) and nonmetals (boron, carbon, nitrogen) in electric furnaces at 1600.degree.-2200.degree. C. during several hours (ibid., pp. 265-293).
Another, more economically advanced and simpler method for making high-melting compounds (U.S. Pat. No. 3,726,643) comprises blending at least one metal selected from groups IV-VI of the periodic table of the chemical elements with at least nonmetal selected from carbon, nitrogen, boron, silicon, oxygen, phosphorus, fluorine, chlorine, and locally igniting the resultant mixture by any appropriate known method, e.g. using a tungsten filament. A temperature is thus provided which is necessary for initiating an exothermal reaction between metals and nonmetals in a small area of the mixture.
Further reaction of components of the mixture does not call for use of external energy supplies for heating and occurs on the account of heat of the exothermal reaction proper. The reaction propagates spontaneously through the mixture in the form of combustion due to heat transfer from a heated mixture layer to a cold layer at a combustion speed of 4 to 16 cm/s.
This method for making hard alloys is a multiple-stage process: it comprises a stage of preliminary preparation of high-melting compounds and their subsequent processing using known powder metallurgy techniques.
In addition, this method is associated with high power requirements and does not allow high-density homogeneous materials based on certain high-melting compounds such as TiB.sub.2 -based compounds to be produced because of their poor sintering capacity.
Known in the art is a method for making a hard alloy material (U.S. Pat. No. 4,431,448) having a porosity of less than 1% of the following composition in % by mass:
______________________________________ titanium diboride 40-60 binder 3-30 tatanium carbide the balance, ______________________________________ comprising blending by mixing powders of titanium, boron, carbon and at least one of metals of subgroup IB of the periodic table of the chemical elements inactive with respect to boron or an alloy based on one of the abovementioned metals or powders of metals forming said alloy under exothermal reaction conditions. The mixture is then compacted and locally ignited to initiate an exothermal reaction between titanium and boron and carbon which then goes on spontaneously under combustion conditions and propagates through the mixture by heat transfer from a heated mixture layer to a cold layer. When the exothermal reaction is over, the resultant solid-liquid reaction mass is pressed to a porosity below 1%.
The abovedescribed method features simplicity of process and cost-effectiveness. Its emploment makes it possible to prepare materials with high hardness and wear resistance with a high enough strength. The use of the abovementioned combination of steps allows porosity of a hard alloy to be lowered to 1% and even lower.
However, the materials prepared by using the abovedescribed method have inadequate uniformity of chemical composition and physico-mechanical properties.
This is due to the fact that completeness of reaction between components of the mixture in the combustion zone is below unity so that combustion products contain unreacted components non-uniformly dictrubuted over the volume of the combustion products.
This results in impaired chemical uniformity of a composite.
At the same time, a disruption of thermal isotropy of the combustion products occurs so that they are non-uniformly compacted thus impairing uniformity of physico-mechanical properties of a composite.
Known in the art is a method for making a hard alloy material (EP No. 165707 A2), comprising blending at least one metallic element of groups III, IVa, Va and VIa of the periodic table of the chemical elements and at least one compound of at least one element from these groups individually or in combination, at least one nonmetallic element selected from the group of boron, carbon, nitrogen and silicon, and at least one compound of at least one of said elements taken individually or in combustion, and a binder component, compacting the mixture, placing the mixture into a synthesis zone, igniting the mixture, with subsequent reaction between the components of the mixture under combustion conditions, and compacting the hot combustion products to obtain an end product.
This method makes it possible to prepare cermets based on various high-melting compounds, the resultant materials featuring high hardness and very low porosity (less than 1%).
Homogeneity of the synthetic materials is, however, inadequate which is due to incomplete reaction between components of the starting mixture, non-uniform compaction of combustion products and substantial thermal load upon the end product.
Non-uniformity of hard alloys produced by the abovedescribed method may result in an instability and even in a material impairment of operating properties of parts made from such materials in certain applications.